Apparatus for preionizing apulsed gas laser

ABSTRACT

An apparatus for preionization of a pulsed gas laser comprises corona preionization electrodes (12, 12&#39;) which are each arranged adjacent a respective associated main electrode (10, 10&#39;). To generate an effective preionization with low constructional and circuit expenditure the preionization electrodes (12, 12&#39;) are set under voltage with the same high-voltage source (16) which also supplies the main electrodes (10, 10&#39;) with voltage. By means of an inductance (30) a time delay is set between the excitation of the preionization electrodes and the triggering of the main discharge between the main electrodes (10, 10&#39;).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an apparatus for preionizing a pulse gas lasercomprising preionizing electrodes each arranged adjacent associated mainelectrodes in order to generate on excitation by means of high voltageultraviolet radiation which prior to a main discharge between the mainelectrodes effects the preionization of the gas, the main dischargebeing triggered by means of high-voltage pulses generated by ahigh-voltage source.

2. Description of the Prior Art

Pulsed gas lasers, for example excimer lasers and CO₂ lasers, can beoperated in accordance with the prior art by socalled transverse pulsedgas discharges (TE gas lasers). This gas discharge (also referred to asplasma discharge or main discharge) takes place perpendicularly to theoptical axis of the laser. The energy necessary for the main dischargeis stored in a capacitor and transferred to the plasma during thedischarge. The plasma discharge usually takes place in the laser chamberbetween main electrodes arranged parallel to the optical axis.

The power and other qualities of the laser depend inter aliasubstantially on the homogeneity of the plasma discharge. To ensure thenecessary homogeneity of the plasma discharge at different pressures ofthe working gas in accordance with the gas mixture, a socalledpreionization is necessary before the plasma discharge (main discharge).The preionization of the gas in the space between the main electrodes ofthe laser is carried out in particular also to avoid arc discharges.

In such a preionizing the gas is ionized in the discharge space betweenthe main electrodes to prepare for the main discharge, i.e. freeelectrons are generated in the gas. Typically, in such a preionizingrelatively low electron concentrations (for example 10⁷ electrons/cm³)are generated in the discharge space. In the main discharge, which takesplace delayed with respect to the preionization, the low initialconcentration of free charges generated during the preionization ismultiplied in a short time via socalled avalanche processes and byionizing processes in the laser gas electron concentrations of 10¹⁴ to10¹⁵ electrons/cm³ are reached.

In the prior art different methods of preionization are known. Usually,ultraviolet radiation is used which is obtained for example by sparkgaps or by corona discharges.

Spark preionization apparatuses involve a considerable constructionalexpenditure, requiring in particular the introduction of a plurality ofinsulated high-voltage passages in gas-tight manner in the laser chamberon both sides along the main electrodes, and furthermore sparkpreionization systems also have the disadvantage that by erosionprocesses in the hot spark plasmas gas impurities arise which impair thelaser performance and in particular the life of the laser.

Generally, spark preionization systems provide a higher electron densityin the discharge gas than corona preionization apparatuses. However,with corona preionization apparatuses as well it is possible to achievea glow discharge sufficient for a pulsed gas laser and of goodhomogeneity, in particular in the case of XeCl excimer lasers and CO₂lasers.

In the corona preionization apparatus ultraviolet light is generated ina gas discharge between a metal and a dielectric. This ultravioletradiation then generates in the gas of the discharge space theaforementioned weak ionization, i.e. the generation of free electronsreferred to. Following this preionization a homogeneous gas dischargecan then be triggered between the main electrodes of the laser.

In a corona preionization apparatus the dielectric prevents theformation of spark channels to the preionization electrodes (which areto be distinguished from the main electrodes in a manner known to theperson skilled in the art). During the preionization only the electricalcapacitance formed from the preionization electrodes and the dielectricis charged. In spite of relatively low currents an intensive emission ofUV light is obtained (G. J. Ernst and A. G. Boer, Opt. Commun. 27, 105,1978; U. Hasson and H. M. von Bergmann, Rev.Sci. Instrum. 50, 59, 1979).

In such a use of dielectrics in the corona preionization sparks areeffectively suppressed and thus also the disadvantages caused by sparks,in particular erosion processes at the electrodes and gas impurities.

The prior art of corona preionization apparatuses contains essentiallytwo types of electrical connection of the corona electrode. Either thecorona electrode is supplied from a separate high-voltage circuit, i.e.the corona electrode has its own high-voltage source independent of themain electrodes, or the corona electrode is connected in simple mannerdirectly to the electrical potential of the counter main electrode. Thisprior art will be explained in detail hereinafter with the aid of FIGS.1 to 4.

FIGS. 1 to 3 show different embodiments of a corona preionizationapparatus in which the corona electrode is connected to the potential ofthe counter main electrode (R. Marchetti and E. Penco, J. Appl. Phys.56, 3163, 1984). In known manner, two main electrodes 10, 10' arearranged opposite each other in the laser chamber. Adjacent the one mainelectrode 10 preionization electrodes 12, 12' are arranged. Each of thepreionization 12, 12' is surrounded by a tubular dielectric (e.g.ceramic) 14, 14'. A high-voltage source known per se is designated bythe reference numeral 16. The high-voltage source 16 charges a storagecapacitor 18. Via a thyratron the gas discharge is switched in knownmanner. For this purpose, in known manner a recharge inductance 22(coil) is provided and discharge capacitors C₁, C₂ are connected inparallel with the main discharge taking place between the mainelectrodes 10, 10'.

In accordance with FIG. 1 the preionization electrodes 12, 12' arrangedadjacent the one main electrode 10 are connected to the potential of thecounter main electrode 10, i.e. the preionization electrodes 12, 12'have the potential of the counter main electrode 10' and due to theirsmaller spacing from the one main electrode 10 a very high fieldstrength arises between the one main electrode 10 and the preionizationelectrodes 12, 12' and generates a corona discharge on the dielectrictubes 14, 14'. The corona discharge in turn emits UV radiation whichpreionizes the gas between the main electrodes 10, 10'.

FIG. 2 shows a modification of the example of embodiment according toFIG. 1, two preionization electrodes 12, 12' now being arranged near thelower main electrode 10' but being connected to the potential of thecounter main electrode 10 so that the corona discharge effecting apreionization burns close to the main electrode 10' drawn at the bottomin the Figures.

In the Figures, corresponding components are provided with the samereference numerals. In FIGS. 2 and 3 the high-voltage source 16, thestorage capacitor 18, the thyratron 20 and the resistor R₁ have not beenillustrated for the sake of simplicity.

A corona preionization apparatus according to the FIGS. 1 and 2 has theadvantage that defects in the dielectric (for example small holes andcracks) can lead to electrical breakdowns (between the preionizationelectrode and the adjacent main electrode) in which the energy of themain discharge can be used up and the dielectric 14, 14' can bedestroyed. Admittedly, such a consumption of the energy of the maindischarge or a destruction of the corona dielectric can be prevented bya capacitive current limiting by means of the capacitors C₃, C₄ ;however, such a capacitative voltage division also leads to a loss ofefficiency in the transfer of electrical energy to the corona discharge.

The separate current supply of the preionization electrodes shown inFIG. 4 by means of a separate switch 26 and a separate high-voltagesource 28 (apart from the high-voltage source 16) requires a relativelygreat constructional expenditure for the switching elements and thesynchronization circuits for the time synchronization of preionizationand main discharge when compared with the socalled automatic orautonomous circuits according to FIGS. 1, 2 and 3 explained above

In the journal "JAPANESE JOURNAL OF APPLIED PHYSICS", Vol. 29, No. 1,January 1990, p. 95-100 (article by K. NAKAMURA et al.) for thepreionization an independent separate circuit is provided (separate withrespect to the voltage supply of the main electrodes). This separatepreionization circuit consists of the elements G₃, C₃, L₃, C_(S) andL_(S).

JP 60-157 280 (A) describes a preionization in which the preionizationelectrodes are driven via an auxiliary capacitor. EP 0 398 330 A2 alsodescribes such an arrangement in which a driving of the preionizationelectrode is effected via an auxiliary capacitor (number 12 in FIG. 4).In these last two systems of the prior art mentioned the coronaelectrode is subjected to high voltage for an unnecessarily long timeand this leads to an increased risk of dielectric breakdown. The priorart according to the last three documents mentioned requires in eachcase a complicated structure of the main electrode (either meshelectrodes or electrodes with buried corona rods).

For laser systems with very high repetition rates and long lives theseknown systems have not proved very suitable.

SUMMARY OF THE INVENTION

The invention is based on the problem of setting forth a simpleapparatus for the preionization of a pulsed gas laser which achieves apreionization of good quality whilst requiring low constructional andcircuit expenditure.

According to the invention this problem is solved in that the excitationof the preionization electrodes is effected with the same high-voltagesource which also charges the capacitors of the main electrodes and bymeans of an inductance a time delay is set between the excitation of thepreionization electrodes and the triggering of the main dischargebetween the main electrodes.

The electrical driving of the preionization electrodes according to theinvention (in a simple modification of the invention a singlepreionization electrode may also be provided) utilizes the knowledgethat the voltage drop at the deliberately inserted or parasiticallypresent inductance in the socalled recharging circuit can be utilized toapply a voltage to the preionization electrodes a short time beforeapplication of the high-voltage pulse to the main electrodes (withrespect to said main electrodes) in such a manner that a goodpreionization is achieved by means of corona discharge. For during thecharging of the discharge capacitors C₁, C₂ a relatively large currentchange occurs and by connecting the preionization electrode before theinductance a very rapidly arising potential difference occurs betweenthe preionization electrodes and the main electrode and thus acorrespondingly intensive corona discharge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 show different embodiments of a corona preionization apparatusin which the corona electrode is connected to the potential of thecounter main electrode.

FIG. 4 shows an embodiment of a corona preionization apparatus having aseparate current supply for the preionization electrodes.

FIG. 5 shows a circuit for the preionization of a pulsed gas laser inwhich the high voltage for the preionization electrodes is tapped off inthe high-voltage pulse direction before an inductance.

FIG. 6 shows a modification of the example of embodiment according toFIG. 5, the higher voltage for the preionization electrodes being tappedoff between two inductances.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter two examples of embodiment of the invention will bedescribed in detail with the aid of FIGS. 5 and 6.

In FIGS. 5 and 6 the components corresponding to those according toFIGS. 1 to 4 are provided with identical reference numerals. Thus,adjacent respective main electrodes 10, 10' preionization electrodes 12,12' are arranged, the spacing of the preionization electrodes 12, 12'from one of the main electrodes being less than from the other mainelectrode. According to the FIGS. 5 and 6 the preionization electrodesare arranged nearer the upper main electrode 10 than the lower mainelectrode 10'. In a modification of the example of embodimentillustrated it is also possible to provide a single preionizationelectrode 12; the symmetrical arrangement according to the Figures ishowever preferred.

Each of the preionization electrodes 12, 12' is surrounded by arespective tubular dielectric 14, 14'. The corona discharge burns on theouter surface of the dielectric.

A high-voltage source known per se for generating high-voltage pulses isindicated by the reference numeral 16. The thyratron 20 alreadydescribed above, the storage capacitor 18 and the recharging inductance22 are omitted in order to concentrate the illustration on the essenceof the invention.

Thus, in the circuits according to the invention for preionization of apulsed gas laser as well for the preionization the same energy source isused which also serves for the main discharge, as explained with the aidof FIG. 1, i.e. the high-voltage source 16 and the energy storage means18 respectively.

The essential matter is the provision of an inductance (coil) 30 in thepreionization and main discharge circuit and the arrangement of the line32 with which the preionization electrodes 12, 12' are put undervoltage.

In the example of embodiment according to FIG. 5 the high-voltage source16 generates high-voltage pulses with which the discharge capacitors C₁,C₂ are successively charged. During the charging of the dischargecapacitors C₁, C₂ great current changes and corresponding voltage dropsoccur at the inductance 30. According to the invention the preionizationelectrodes 12, 12' are supplied with voltage via a line 32 which tapsthe potential off in the charging direction of the high-voltage pulsesbefore the inductance 30. Due to this voltage tapping in front of theinductance 30, a very rapidly starting potential difference occursbetween the preionization electrodes 12, 12' and the main electrode 10adjacent thereto and correspondingly a corona discharge occurs on thedielectrics 14, 14' surrounding the preionization electrodes 12, 12'. Asregards time the corona discharge thus generated coincides with thevoltage rise between the main electrodes 10, 10'. The inductance 30 isso dimensioned that the time delay between the starting of the coronadischarge and the main discharge gives a maximum laser power. For agiven laser system the dimensioning of the capacitances and inparticular of the inductance 30 depends on the specific linearrangements and the resulting parasitic inductances and capacitancesand must be determined experimentally for the particular individualcase. For a great number of excimer laser gas mixtures the time delaybetween preionization by means of corona discharge and subsequent maindischarge has proved to be an optimum one for achieving a high laserpower and long laser life. Special synchronization circuits are notnecessary.

A further advantage of the circuit arrangement according to theinvention for a preionization resides in that with increasing chargingof the discharge capacitors C₁, C₂ the potential difference between thepreionization electrodes and the associated main electrode diminishes.The voltage pulse obtaining between said electrodes is therefore ofrelatively short duration so that the risk of a breakdown through thedielectric is considerably reduced. As dielectric, in particular Al₂ O₃ceramic or sapphire have proved suitable.

FIG. 6 shows a modification of the example of embodiment according toFIG. 5, the inductance 30 being replaced by two inductances 30a and 30band the line 32 applying the voltage to the preionization electrodes 12,12' tapping the voltage between the two inductances 30a, 30b. Bysuitable variation of the inductances 30a, 30b the voltage profilebetween the preionization electrodes and the main electrode can be setexperimentally and optimized for a specific system.

The arrangement described above is very simple in construction andcircuitry and has proved itself for a great number of excimer laser gasmixtures; in particular, with XeCl and KrF laser gas mixtureshomogeneous glow discharges up to very high pulse repetition rates of200 Hz were implementable. The efficiency of the emitted laser radiationwas exactly as high as when using a substantially more complicated sparkpreionization. The preionization apparatus according to the inventionrequires only four high-voltage leadthroughs into the laser chamber.

The circuit arrangements described above with the aid of FIGS. 5 and 6for preionization of a pulsed gas laser drive the preionizationelectrodes 12, 12' utilizing the inherent capacitance of said rodelectrodes. Additional capacitors and switches for operating thepreionization are superfluous.

In addition, the arrangements described ensure an only short voltagepulse at the preionization electrodes, this leading on the one hand toan intensive corona discharge and on the other to a reduced risk ofelectrical breakdown of the dielectric. The geometry of the maindischarge electrodes and the preionization electrodes described abovewith the aid of the Figures and the circuit described permit the use ofsolid profile main electrodes 10, 10'. Such solid profile mainelectrodes (i.e. solid electrodes in the form indicated in FIGS. 4 and5) have advantages as regards the erosion behaviour and the control ofthe discharge cross-section. Apart from the advantages referred to abovethe circuit arrangement according to the invention also permits acompact structure.

What is claimed is:
 1. A pulsed gas laser comprising:first and secondmain electrodes which are arranged in a chamber containing a gas; firstand second preionizing electrodes arranged adjacent said main electrodesfor generating ultraviolet radiation which preionizes said gas betweensaid main electrodes prior to a main discharge between said mainelectrodes; a first electrical circuit for generating said ultravioletradiation including said preionizing electrodes and an electrical linefor applying a voltage to said preionization electrodes; a secondelectrical circuit including said main electrodes for facilitating saidmain discharge between said main electrodes; a switching element forclosing both said first electrical circuit including said preionizingelectrodes and said second electrical circuit including said mainelectrodes; a capacitor common to both said first electrical circuit andsaid second electrical circuit which is connected in series with bothsaid first and said second electrical circuit such that the charge ofsaid capacitor is utilized for both a preionization of said gas and forsaid main discharge; a high voltage source for charging said capacitor;and, an inductance connected in series with said second electricalcircuit including said main electrodes for setting a time delay betweensaid preionization and said main discharge.
 2. The pulsed gas laser ofclaim 1, wherein said first electrical circuit and said secondelectrical circuit additionally comprise a second inductance whereinsaid electrical line of said first electrical circuit taps the voltagebetween said two inductances.
 3. The pulsed gas laser of claim 1,wherein said preionization electrodes are corona electrodes.
 4. Thepulsed gas laser of claim 3, wherein said corona electrodes aresurrounded by a tubular dielectric.